In all fluorescent lamps, the mercury arc in the lamp generates primarily short wave (SW) ultraviolet (UV) energy at approximately 253.7 nm. When a lamp is manufactured, a fluorescent phosphor is applied to coat the inside of the lamp (bulb wall), which is usually made of a soda-lime glass. When the lamp is operating, the SW ultraviolet light hits the phosphor on the inside of the lamp causing the phosphor to fluoresce in the visible spectrum that we see (usually some type of white color). The type of phosphor coating determines the color of the fluorescent light emitted.
Some special UV fluorescent type lamps, instead of having a phosphor that fluoresces primarily in the visible light (white), they have a phosphor that fluoresces primarily in another UV wavelength. These UV phosphors are of different compounds or mixtures depending on the peak UV spectral distribution output desired, e.g., 306, 312, 351, or 368 nm. Other UV spectral distributions are also available based on mixtures of the different UV phosphors.
In some special UV fluorescent lamps, the UV fluorescence will not effectively transmit through the soda-lime glass bulb, so a different, higher UV-transmitting glass is used, such as erythemal glass. Also in some UV lamps, quartz is used instead of erythemal or soda-lime glass.
For many UV fluorescent applications, a SW filter is inserted over the UV lamp to block the small amount of visible light generated by the UV arc and phosphor in the lamp. This normal SW filter has a very broad UV band-pass. It transmits most of the UV wavelengths desired but still blocks most of the visible light. Since many of the fluorescent applications are done in the dark, the small amount of visible light generated by the SW lamp would mask or override the fluorescent effect without an ultraviolet-transmitting visible-absorbing filter.
With these current devices, the only way to change wavelengths is to open up the light assembly and change the lamp bulb. Not only does this present a problem for the typical UV light user, but it necessitates that a field geologist, mineralogist, or amateur collector carries several different UV light assemblies into the field.
U.S. Pat. No. 6,670,619 to Waluszko discloses a transilluminator having conversion means held between such a SW lamp and a light-absorbing filter. However, Waluszko's conversion means are specifically a plate with a phosphor coating applied thereon. The conversion means of the current invention are polymer or plastic sheets which have the specific UV phosphor mixed in with the polymer or plastic during manufacture. The resulting conversion sheets are more lightweight and flexible than plates with coatings and are applicable to handheld devices.